In recent years, much research has been being carried out into forming an insulated gate type semiconductor device (MISFET) on an insulating substrate or on a surface separated from a semiconductor substrate by a thick insulating film (an insulating surface). In particular, a semiconductor device in which a semiconductor layer (active layer) is of a thin film form is called a thin film transistor (TFT). In such a semiconductor device, it is difficult to obtain an element having such good crystallinity as that of a single crystal semiconductor, and usually a non-single crystal semiconductor which is not single crystal but has crystallinity has been employed.
Such a non-single crystal semiconductor has inferior characteristics as compared with a single crystal semiconductor. In particular, there has been the problem that when a reverse voltage (that is, a negative voltage in case of an N channel type TFT and a positive voltage in case of a P channel type TFT) is applied to gate electrodes, a leak current between a source and a drain is increased. This problem has been fatal particularly when a TFT is used for a switching transistor of an active matrix circuit.
It has been reported that this problem can be solved by thinning a semiconductor layer (active layer) in which channels are formed in a TFT. For example, Hisao Hayashi et al report in Jpn. J. Appl. Phys. vol. 23 (1984) L819 that they studied how the characteristics of a TFT were affected when the thickness of an active layer of crystalline silicon was changed from 100.ANG. to 1000.ANG. and obtained the desirable characteristic that as the active layer gets thinner, electric field effect mobility increases, and threshold voltage and leak current decrease.
According to this report, however, the electric field effect mobility was very low, 10 cm.sup.2 /Vs at maximum, and accordingly while the above TFT could be used for a switching transistor for an active matrix circuit, it was impossible to use the TFT for a circuit for driving the active matrix circuit. According to this report, a film obtained in an as-depo state was utilized for a crystalline silicon film, and it did not have preferred crystallinity.
On the other hand, a method in which crystal growth is effected by heat annealing (solid phase growth method, SPC) and a method in which crystallization is carried out through a liquid phase state or in a solid phase by irradiating with a laser or strong light equivalent to a laser (photo annealing) has been known as methods for obtaining a semiconductor film having good crystallinity from a nonsingle crystal semiconductor. For example, to obtain a silicon film from non-single crystal silicon by heat annealing it has been necessary to heat an amorphous silicon film at 500.degree. to 650.degree. C.
However, because of influences exerted by the substrate (including a base), it has not been possible to obtain good crystallinity by these methods without using a silicon film having a thickness of at least 500.ANG..